The present invention relates measuring the impulse response of a system, and in particular, reducing errors when averaging multiple repetitions to smooth the impulse response measurement of phone systems.
Known methods have difficulty in measuring the frequency response of a Device Under Test (DUT), for example a phone, clocked separately from a measurement system. Although the clocks of the phone and the measuring device may vary by as little as 0.001%, methods averaging multiple measurements for noise reduction may fail. For example, when measuring the frequency response of a DUT that is playing back at 48 kHz, with a test system that is recording at 48 kHz, the two sampling rates will differ from each other by a very small amount, due to the relative precision of their individual oscillators. When measuring acoustic signals, averaging multiple repetitions of a frequency response measurement signal is performed in the digital domain to reduce the measured and uncorrelated noise. When the sampling rates differ even slightly, each repetition of the frequency response measurement signal becomes more and more out of phase with the DUT signal, until cancellation and comb filtering arise.
Measurements might be performed using a single measurement iteration, but no noise reduction would result.